Voice Over Internet Protocol (VOIP) is a packetized and digitized transmission technology based on the Internet protocol (IP) technology. The basic principle of VOIP is as follows: compressing and encoding the voice by voice compression algorithm, then packaging the compressed voice data according to relevant protocol(s) (e.g., IP protocol), then transmitting the packages to a destination via the IP network, then connecting these voice data packets in series, and restoring these voice data packets to the original voice signal after decoding and decompressing process, so as to achieve the purpose of transmitting voice over IP network.
An IP Phone System transforms analog signals of an ordinary phone into IP packets which can be transmitted by a computer through accessing the Internet; meanwhile, it also transforms the received IP packets into analog electronic signals of the voice. The application of voice communication between a computer network and the fixed network by using VOIP software in a personal computer (PC) has been well developed.
With the rapid development of mobile communications, due to the increase of equipment capacity and the network bandwidth of a mobile terminal, it is possible to realize a real-time voice data intercommunication between a computer network terminal and a mobile communication network terminal and between mobile communication network terminals. By using VOIP technology, IP phone applications can be carried out on an IP-based data communication network, such as the enterprise Local Area Network (LAN), Wide Area Network (WAN), Worldwide Internet and so on. Making the international and domestic long-distance call by IP phone has low cost, which reduces communication costs, and ensures a good voice quality at the same time. With the development of the network communication and wireless technology, a mobile phone can simultaneously support multiple wireless technologies for accessing a telephone service network. With the support of the multiple wireless technologies, a mobile phone can support VOIP session function, for example, a mobile phone supports General Packet Radio Service (GPRS), Enhanced Data Charge for GSM Evolution (EDGE), Wireless LAN based on the IEEE 802.11b standard (Wireless Fidelity, abbreviated as WIFI), Worldwide Interoperability Network for Microwave Access (WIMAX), and it accesses an IP link of the core network via these networks, so as to provide VOIP services. As the coverage construction, charges and quality of service (QOS) differ in different wireless networks, during communicating via a wireless network, if a user departs from the current wireless network coverage, or wants to switch to another wireless network, it leads to a session interruption. Therefore, there is a need for a seamless switching method which can ensure that the originally initiated session will not be interrupted when the user switches among different wireless networks, and that the switch can be freely realized.